The present invention relates to an integrated optical element and a method for manufacturing an integrated optical element.
An integrated optical element is, for instance, achieved by forming a plurality of functional areas where specific types of processing are implemented on a guided wave light so that the wave guide channels at the individual functional areas are connected to each other on, for instance, a substrate. Integrated optical elements formed in this manner in the conventional art include the light modulator integrated laser disclosed in an article entitled "High-Speed(20 Gbit/s), Low-Drive-Voltage (2Vp-p)Strained-InGaAsP MQW Modulator/DFB Laser Light Source" in the Electronic Communication Society Monogram Collection C-1 Vol. J77-C-1 No. 5 pp. 268-275, May, 1994. In this light modulator integrated laser, a DFB laser (distributed feedback laser) area and a modulator area are formed as functional areas.
In a light modulator integrated laser in the conventional art, the wave guide channel in the laser area is formed to achieve an embedded wave guide channel structure so that a low threshold value, single mode oscillator and a higher output are achieved through sufficient light trapping. In addition, the wave guide channel in the modulator area in the light modulator integrated laser in the prior art is formed to achieve a so-called high mesa structure in order to realize high speed modulation through a lowered element capacity.
It is to be noted that a wave guide channel in a semiconductor optical element normally adopts a laminated structure which is achieved by sequentially laminating a first clad layer, a light wave guide layer which is to constitute the active core and a second clad layer on a substrate in order to achieve light trapping in the longitudinal direction. Moreover, in the wave guide channel of the semiconductor optical element, a mesa structure is formed from several layers at the side where the second clad layer is formed in this laminated structure, under normal circumstances. The mesa structure in this context refers to a structure in which a cross section perpendicular to the direction in which the guided wave light is transmitted forms a mesa (trapezoidal shape), which may be formed through, for instance; etching, selective growth or the like.
The high mesa structure, which is adopted in the modulator area of a light modulator integrated laser in the prior art refers to a structure in which the light wave guide layer is included in the mesa structure. A structure in which the light wave guide layer is not included in the mesa structure, in contrast, is referred to as a low mesa structure.
In order to manufacture the light modulator integrated laser in the conventional art disclosed in the publication above, first, a multiple quantum well (MQW) layer that is to constitute a light wave guide layer is grown on a substrate on which the first clad layer has been formed, in a portion that is to constitute the modulator area, the portion that is to constitute the laser area and the area that is to constitute a separation area which is located between the two areas mentioned earlier.
Next, etching is performed down to the first clad layer on the portion that is to constitute the layer area while leaving the multiple quantum well layer over a width of approximately 1.5 micrometers and etching is performed down to the first clad layer on the portion that is to constitute the modulator area and the portion that is to constitute the separation area while leaving the multiple quantum well layer over a width of approximately 30 micrometers. Then, in order to achieve lateral light trapping in the multiple quantum well layer, both sides of the remaining multiple quantum well layer are filled with InP, forming p-n junctions. In addition, in order to achieve longitudinal light trapping, a laminated structure is formed by regrowing a clad layer constituted of p-InP and a contact layer, thereby forming an embedded type wave guide channel in the laser area.
Then, etching is performed on the portion that is to constitute the modulator area and the portion that is to constitute the separation area down to the first clad layer while leaving them unetched over widths of approximately 2.5-3 micrometers (&lt;approximately 30 micrometers) and on the laser area down to the first clad layer by leaving it unetched over a width of approximately 220 micrometers (&gt;1.5 micrometers). Through this etching, a wave guide channel having a high mesa structure is formed in the modulator area.
In the light modulator integrated laser in the conventional art which is manufactured as described above, a laser area having a core over a width of approximately 1.5 micrometers and a modulator area having a core over a width of approximately 2.5-3 micrometers are formed with a separation area having a core over a width of approximately 2.5-3 micrometers formed there between.
FIG. 11 presents a graph of the results of calculation of the relationship between the quantity of misalignment x between the cores and the coupling efficiency y of the guided wave light achieved when two wave guide channels are connected. The calculation is performed on the assumption that the two wave guide channels have cores over the same width W. It is learned from FIG. 11 that the coupling efficiency decreases as the width of the cores becomes smaller.
This means that the coupling efficiency tends to become reduced greatly and the alignment of the wave guide channels is extremely difficult in a structure in which wave guide channels having cores over a small width are connected with each other as in the light modulator integrated laser in the prior art. FIG. 11 indicates that on the scale of the light modulator integrated laser in the prior art, the coupling efficiency y becomes reduced down to approximately 0.5-0.6 even with the misalignment quantity x at approximately 1 micrometers.
In other words, since the coupling loss greatly increases if the alignment of the wave guide channels becomes even slightly offset in an integrated optical element in the conventional art, it is necessary that high precision in etching mask alignment be achieved during the manufacturing process. In addition, when an integrated element in the conventional art is miniaturized, it may result in a poor production yield of the element.